Heteropolyacids Wacker process

In the group of Backvall a method was developed involving palladium and benzoquinone as cocatalyst (Fig. 4.42) [103]. The difficulty of the catalytic reaction lies in the problematic reoxidation of Pd(0) which cannot be achieved by dioxygen directly (see also Wacker process). To overcome this a number of electron mediators have been developed, such as benzoquinone in combination with metal macrocycles, heteropolyacids or other metal salts (see Fig. 4.42). Alternatively a bimetallic palladium(II) air oxidation system, involving bridging phosphines, can be used which does not require additional mediators [115]. This approach would also allow the development of asymmetric Pd-catalyzed allylic oxidation. 

The future of the commercial acetaldehyde processes mainly depends on the availability of cheap ethylene. Acetaldehyde has been replaced as a precursor for 2-ethylhexanol ( aldol route ) or acetic acid (via oxidation cf. Sections 2.1.2.1 and 2.4.4). New processes for the manufacture of acetic acid are the Monsanto process (carbonylation of methanol, cf. Section 2.1.2.1), the Showa Denko one-step gas-phase oxidation of ethylene with a Pd-heteropolyacid catalyst [75, 76], and Wacker butene oxidation [77]. Other outlets for acetaldehyde such as pentaerythritol and pyridines cannot fill the large world production capacities. Only the present low price of ethylene keeps the Wacker process still attractive. 

Heteropolyacids can be very useful in oxidation reactions. In contrast with metal chelate catalysts that usually become oxidized and deactivated eventually, the heteropolyacids are extremely stable to oxidation. Acetaldehyde is produced commercially from ethylene by the Wacker reaction with a palladium(II) chloride catalyst, copper(II) chloride, oxygen, and water. The corrosive conditions are a disadvantage of the process. Catalytica Inc. has devised a process (6.55) that uses only 1% as much palladium and chloride as the usual process. It uses a small amount of palladium (II) chloride with a partial sodium salt of phosphomolybdovanadic acid.287... 

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